Bipolar stepper stalls for high values of current

[amelia]

Hello again,
I am using the 3333_0 57STH56 NEMA23 Bipolar Stepper with 15:1 Gearbox and the 1067 controller. I am using the stepper installed in a machine to lift weight at a extremely low speed, the motor stalls even there is no load.
After this strange behaviour, I did some tests with the Phidget Control Panel to check the velocity values when the motor stalls and its relation with the current. The motor is alone (unistalled from the machine, just on my desk) with no load at all.
TEST 1.1
- Position = a very high value to maintain the stepper working during the test
- Velocity limit = 50.000 microsteps/s
- Acceleration = 64 microsteps/s^2
- Current = 2.8A
Result: The motor stalls at about 30.000 microsteps/s
TEST 1.2
- Velocity limit = 50.000 microsteps/s
- Acceleration = 64 microsteps/s^2
- Current = 2A
Result: The stepper does not stall. It works fine
TEST 2.1
- Velocity limit =125 000 microsteps/s
- Acceleration = 1024 microsteps/s^2
- Current = 2.8A
Results: The stepper stalls atabout 30.000 microsteps/s
TEST 2.2
- Velocity limit =125 000 microsteps/s
- Acceleration = 1024 microsteps/s^2
- Current = 2A
Results: The steppers does not stall.

I have obtained similar results for other values of acceleration and current. On the contrary that the User guide says, it seems that the stepper stalls for high values of current even for higher values of acceleration.

Could you help me please?

Thanks!

[Bob-666]

What is the voltage and current rating of the motor power supply?